1. Field of the Invention
This invention relates generally to a process for applying spin-glass to a substrate, and more particularly to a process for the planarization of semiconductor wafers. The invention is especially applicable to inorganic spin-on glasses.
2. Description of the Prior Art
Spin-on glasses (SOG) are proprietary liquid solutions containing siloxane or silicate based monomers diluted in various kinds of solvents or alcohols. They are commonly used for the planarization of semiconductor wafers, i.e. the filling and levelling of the trenches formed between interconnect paths deposited on the wafer. On coating and curing of spin-on glasses, monomers are polymerized by condensation and release of water, solvent, and alcohol. The condensed material is a thin solid film having mechanical, chemical and electrical properties which depend on the starting solution, and the coating and curing process.
There are more than one hundred different SOG solutions currently available. These are classified into two major families:
1) Inorganic silicates. PA1 2) Quasi inorganic siloxanes (methyl-siloxanes, ethyl-, phenyl-, butyl-). PA1 (i) the spin-on glass is applied to the wafer in a coating and spinning chamber in a moisture-free environment; PA1 (ii) the wafer is transferred in a moisture-free environment to a curing station; PA1 (iii) the spin-on glass is cured still in a moisture-free environment; and PA1 (iv) steps (i) to (iii) are repeated until a sufficient film thickness has been achieved without in the interim exposing the wafer to ambient conditions such that reverse hydrolysis is minimized during the planarization process. PA1 a) The wafer is transported from sending cassette to a coating chamber. PA1 b) a few ml of a SOG solution are dispensed at the centre of the wafer to be planarized. PA1 c) The wafer is spun at a given RPM to spread uniformly the solution and to permit the evaporation of volatile compounds and film solidification. PA1 d) For SOG curing, the wafer is sequentially transported to in-line hot plates which are temperature controlled at temperature roughly between 80.degree. C. and 250.degree. C. PA1 e) The wafer is slightly cooled at an idle station. PA1 f) The wafer is stored and cooled in a receiving cassette. PA1 g) When all the wafers are received in the receiving cassette, they are all together transferred to the sending cassette for a second coat (steps a to f are repeated). Such a transfer can be carried out in any standard laboratory transfer chamber. When all the wafers are received in receiving cassette, they are all together transferred to the sending cassette for a third coat (steps a to f are again repeated). PA1 h) When sufficient coats have been applied, the wafers are transferred to the station for the next process step.
The difficulty of using silicate SOGs for the planarization of low-melting point materials, such as aluminum has caused a worldwise trend not to use purely inorganic silicate (including phosphosilicate) SOGs and to use instead members of the quasi-inorganic siloxane family.
However, there are significant disadvantages in using quasi-inorganic SOGs relating to their electric properties, and their use is becoming uncertain for advanced applications.
A further problem is caused by the absorption of water during curing. SOGs, particularly phosphorus alloyed SOGs, are extremely hygroscopic materials, and rapidly absorb ambient moisture during curing. This moisture pick-up promotes the irreversible reactions described above, and the resulting SOG films have poor properties and reliability.